Nanonewsletter No

nanonewsletter No. 28 /// December 2013 www.phantomsnet.net AtMol Special Issue

Electron tunnelling manipulation of self- assembled molecular nanostructures Coronene: A Single Molecule Atom Counter Measuring the conductance of single molecular wires as a function of the electron energy Synthesis of Y-Shaped Polyarenes, Crushed Fullerenes and Nanographene Fragments

dear readers, Dear Readers,

The AtMol Integrated Project (EU/ICT/FET) is currently establishing a comprehensive process flow for fabricating a molecular chip, i.e. a molecular processing unit comprising a single molecule connected to external mesoscopic electrodes with atomic scale precision and preserving the integrity of the gates down to the atomic level after the encapsulation.

This E-nano Newsletter special issue contains four articles providing new insights on this hot research topic in Europe, in particular for the fabrication of future generations of electronic circuits with components miniaturized down to atomic scale.

Detailed information about the project, further reading documents and AtMol workshop series contributions could be found at www.atmol.eu.

We would like to thank all the authors who contributed to this issue as well as the European Commission for the financial support (ICT/FET FP7 AtMol project no. 270028).

> Dr. Antonio Correia Editor - Phantoms Foundation

contents 05 > nanoresearch - AtMol. Electron tunnelling manipulation of self-assembled molecular nanostructures /// F. Moresco, A.Nickel, R. Ohmann, J. Meyer, M. Grisolia, C. Joachim and G.Cuniberti 13 > nanoresearch - AtMol. Coronene: A Single Molecule Atom Counter /// C. Manzano, W. H. Soe, M. Hliwa, M. Grisolia, H. S. Wong and C. Joachim 21 > nanoresearch - AtMol. Measuring the conductance of single molecular wires as a function of the electron energy /// M.Koch, F. Ample, C. Joachim and L. Grill 29 > AtMol publications. Further reading 33 > nanoresearch - AtMol. Synthesis of Y-Shaped Polyarenes, Crushed Fullerenes and Nanographene Fragments /// P. Calleja, R. Dorel, P. Mc Gonigal, P. de Mendoza, and A. M. Echavarren

editorial information

No. 28 December 2013. Published by Phantoms Foundation (Spain) editor > Dr. Antonio Correia > [email protected] assistant editors > Maite Fernández, Conchi Narros and José Luis Roldán. 1500 copies of this issue have been printed. Full color newsletter available at: www.phantomsnet.net/Foundation/newsletter.php For any question please contact the editor at: [email protected] editorial board > Adriana Gil (Nanotec S.L., Spain), Christian Joachim (CEMES-CNRS, France), Ron Reifenberger (Purdue University, USA), Stephan Roche (ICN-CIN2, Spain), Juan José Sáenz (UAM, Spain), Pedro A. Serena (ICMM-CSIC, Spain), Didier Tonneau (CNRS-CINaM Université de la Méditerranée, France) and Rainer Waser (Research Center Julich, Germany).

deadline for manuscript submission depósito legal printing Issue No. 29: March 31, 2014 legal deposit Gráficas Issue No. 30: April 30, 2014 BI-2194/2011 Indauchu 333

nano research

Electron tunnelling manipulation of self-assembled molecular nanostructures

Francesca Moresco1, Anja Nickel1, Robin controlled at the atomic scale, can be Ohmann1, Joerg Meyer1, Maricarmen tested by adsorbing specially designed 2 2 Grisolia , Christian Joachim and molecules on a surface in ultra clean 1 Gianaurelio Cuniberti conditions, imaging and manipulating

1 Institute for Materials Science, Max Bergmann them using the tip of an STM.

Center of Biomaterials, and Center for Advancing Electronics Dresden, Dresden, Germany In the last few years, the manipulation of 2 Nanosciences Group & MANA Satellite, CEMES- a molecule with the tip apex of a CNRS, Toulouse, France scanning tunneling microscope (STM) has become a well-established technique

Several methods were developed in the to study the mechanics of a single molecule on a surface [1-3]. last decades to manipulate molecules using the tip of a scanning tunneling microscope. Single atoms and molecules can be moved on a surface in a controlled way. However, for the development of devices at the nanoscale, it is important to move not only adsorbates one by one, but also structures composed by several molecules. Therefore, a purely electronic excitation method was recently used for the controlled movement of weakly interacting assemblies of few molecules. The adsorption of Acetylbiphenyl Fig. 1 > Schematic representation of the electron molecules on a Au (111) surface and tunnelling manipulation of a windmill nanostructure./ the formation of nanostructured given by four molecules and stabilized by The controlled manipulation of single hydrogen bonding will be described in atoms and molecules on metal surfaces this article. The electron tunnelling by means of the STM tip was developed induced manipulation of these during the last two decades [4-6]. In nanostructures will be presented. particular, STM has proven to be a Depending on the polarity and position versatile tool for the atomic-scale of the applied voltage, the direction of manipulation and the construction of movement can be selected. nanostructures [1, 2, 4-7]. Single atoms and molecules can be precisely positioned by lateral [6] or vertical manipulation [8]. Introduction Different mechanisms of manipulation

To develop molecular machines based were developed on metal [6], ultrathin on single molecules, scanning tunneling insulator [9, 10], or semiconductor [11] microscopy (STM) at low temperature, surfaces. Other manipulation techniques with its high resolution and stability, is a employ electric fields [12], or inelastic very convenient technique. Prototypes of tunneling electrons [13]. Voltage pulses molecular machines, which can be were used to desorb or dissociate 555

molecules, to induce conformational through a motion of the molecule, or a changes or chemical reactions [14]. conformational change, or a modification of the electronic structure [13]. In particular, inelastic electron tunneling project

induced manipulation is a process where tunneling electrons are injected from the STM tip positioned above the adsorbed molecule, thus moving it. The electron AtMol energy is normally transferred through an excited state, leading to excitation (rotational, vibrational, or electronic), the rate of which is controlled by the applied bias and the tunneling current [3].

In an STM experiment, when an electron tunnels through an adsorbate, inelastic electron scattering may induce elementary electronic or vibrational excitations of the adsorbate [2]. The inelastic electrons transfer energy to the adsorbate. Such energy, stored in the electronic or

vibrational excitation of the molecule, can trigger movement or chemical Fig. 2 > Top panel: Molecular structure of the transformation at the atomic scale [2]. Acetylbiphenyl molecule used in the described Therefore, electron tunnelling induced experiments. Bottom panel: STM image of a single Acetylbiphenyl molecule on Au (111) stabilized by a manipulation might be based on two defect./ different excitations depending on the applied bias voltage. At high voltages an Electron tunneling was recently used to electron may transiently occupy an induce the controlled rotational switching electronic excited level of the adsorbate; of a molecular motor [15]. A molecular at low voltages vibrational modes may be motor adsorbed on a gold surface could excited. be rotated in a clockwise or anticlockwise direction by selective inelastic electron Experimentally, electron tunnelling tunnelling through different subunits of the induced manipulation is realized by motor. positioning the STM tip above the location of the molecule at a fixed height. The Electron tunnelling induced manipulation feedback loop can be switched off and the has the advantage to be triggered just by tunnelling current and/or the voltage are electronic excitations and to avoid any increased from imaging to manipulation direct mechanical interactions of the STM parameters. The electrons can be injected tip apex with the adsorbate. Moreover, either from the tip or from the substrate being mechanically less destructive than depending on bias polarity. The point in common lateral manipulation, it can be time at which a change in the molecule successfully applied to the controlled happens is recorded by monitoring the manipulation of weakly bonded tunnelling current versus time. A sudden supramolecular assemblies [16]. Recently, change in current is indicative of the it was demonstrated that it is possible to success of the manipulation. These manipulate in a controlled way sudden jumps are caused by changes in supramolecular structure by electron the apparent height of the molecule either tunnelling induced manipulation [16]. 666

In this article, we will describe the adsorption of Acetylbiphenyl molecules on the Au (111) surface and the formation of windmill nanostructures studied by STM at low temperature. In project Fig. 1 the voltage pulse manipulation of the windmill structure is schematically

shown. By applying voltage pulses just AtMol on one selected molecule of the structure, it is possible to move the complete supramolecular assembly to a chosen position on the surface without destroying it.

Experimental details

The 4-Acetylbiphenyl (ABP) molecules (Fig. 2) were studied on the Au (111) surface with a low-temperature STM (5 K) under ultra-high vacuum conditions Fig. 3 > Top panel: STM image of an island of (<10-10 mbar). The gold single crystal disordered ABP molecules on Au (111). Bottom surface was cleaned by repeated cycles panel: Linescan recorded over one of the island of Ne ion sputtering and subsequent molecule, as indicated by the arrow in the top panel./ annealing at 720 K. The molecules were deposited from a Knudsen cell at 320 K tip height was recorded. A jump in the tip onto the Au (111) substrate kept at room height indicates a manipulation event. temperature. After deposition, the Typical voltage pulses between 2 V and sample was cooled down to cryogenic 2.5 V of both polarities were applied for temperatures and transferred to the manipulation. STM. For topographic imaging, either low Results and discussion voltages (0.1 V) or low currents (25 pA) were used to ensure that no Adsorption of Acetylbiphenyl on Au (111) manipulation happens during scanning. Differential conductance spectroscopy In Fig. 2 the chemical structure and a measurements were performed using STM image of a single Acetylbiphenyl lock-in detection with a modulation molecule are shown. frequency of 833 Hz and a modulation The molecule is composed of two phenyl amplitude of 20 mV. To ensure the rings and an acetyl group. When recording of a stable spectrum, the deposited on Au (111) at submonolayer measurements have been done under coverage, the molecules diffuse to step closed feedback-loop conditions with a edge and defects or self-assemble in low set-point current (25 pA). small ordered structures. An STM image Manipulation was performed by applying of a single molecule adsorbed at a defect voltage pulses to a single molecular is presented in the bottom panel of Fig. 2. nanostructure. The STM tip was When the coverage increases, island of precisely positioned on a nanostructure molecules are formed, where disordered and a voltage pulse between tip and (Fig. 3) and ordered structures (Fig. 4) sample is applied. During the pulse, the can be found on the surface. feedback loop was kept closed and the 777

geometry was first optimized using the ASED+ semi-empirical molecular mechanics technique, which is well adapted for such large supramolecular project adsorbates [17]. Constant current images of the optimized geometry were then calculated using ESQC (electron-

AtMol scattering quantum chemistry) [18].

The so obtained calculated geometry is presented in Fig. 5. The calculations confirms that the windmill nanostructures are kept together by a hydrogen bond-like Fig. 444 >>> STM topography image of an island of bonding. The calculated structure shows a self-assembled tetramers of ABP on Au(111). Both chirality can be observed in the same island./ bond length of 3.1 Å [16], in good agreement with the STM topography As one can see in Fig. 2 and in the measurements shown in Fig. 4. linescan at the bottom of Fig. 3, the molecule shows two lobes, one larger and more intense than the other. As confirmed by calculations [16], the more intense lobe corresponds to one phenyl ring connected to the acetyl group, while the other lobe corresponds to the second phenyl ring.

In the ordered islands, the ABP molecules self-assemble into small weakly bonded supramolecular structures formed by ABP tetramers (ABP4) and having the form of windmills.

In Fig. 4, a detail of an island composed Fig. 555 >>> Calculated STM image of a single by ABP windmill nanostructures is shown. windmill nanostructure on the Au (111) surface./ The windmills are present on the surface in two mirror symmetric forms as a To perform electron tunnelling induced consequence of the prochiral nature of the manipulation, it is especially important to single molecules. The islands show determine the electronic structure of a windmills of both chiralities. The windmills windmill structure, as the manipulation can interact each other only very weakly. be induced by tunnelling electrons into the electronic resonances of the molecular By reducing the molecular coverage, nanostructure. single windmills structures can be observed by STM on the clean Au (111) Differential conductance spectra (dI/dV terraces, with enough space in between spectra) have been recorded over single to allow manipulation experiments. ABP4 windmills at several positions. The results are shown in the spectra of Fig. 6. The exact adsorption conformation of a In the inset, the exact position of the STM windmill structure was determined by tip on the windmill during the experiment calculating the STM image of an ABP4 is shown. The experiment was repeated molecular windmill on the Au (111) for three different positions. In all three surface. The molecular adsorption cases, a resonance at a positive voltage 888

of about 2.5 V is observed. This peak can molecule on a surface. In the last few be assigned to the lowest electronic years, mechanically driven molecular excited state of the ABP adsorbate [19]. machines have been demonstrated For negative voltages, the spectra appear including molecular gears [20, 21], motors flatter. However, a broad resonance at [22-24], wheels [25, 26], and different project about -2.2 V can be distinguished in the kinds of nano-vehicles [27, 28]. Moreover, spectrum in all three cases. the manipulation of a molecule with the

STM tip apex has become a well- AtMol established technique to study the mechanics of a single molecule on a surface [1, 2].

On metal surfaces, the standard method for lateral manipulation of single atoms [4], single molecules [1], or small groups of molecules [29] is to bring the tip of the STM close to the adsorbate, thereby increasing the tip adsorbate interaction, and then move the tip to the desired final location on the surface.

Fig. 666 >>> dI/dV spectra taken on the windmill Attempts to move the windmills using this structure. The inset shows a STM image of the conventional lateral manipulation windmill where the exact position of the tip during technique resulted in a destruction of the the measurements is indicated. Corresponding spectra and positions have the same color./ supramolecular nanostructure and the separation of its ABP components. In By comparing the spectra taken at the order to move the individual windmills, three different position on a single voltage pulses of about 2.3 V should be molecule of the windmill (Fig. 6), no applied on top of or close to one of the differences in the measured features was ABP molecules of the nanostructure. As a observed. The large and less intense consequence, a controlled collective feature at negative voltage appear movement of all ABP molecules of the unchanged. For positive voltages, the chosen windmill at the same time could be position of the resonance peak is not observed. changed, but its intensity decreases by moving the STM tip from the center of By positioning the tip on the center of the the windmill to the side. The electronic windmill, a rotation of the complete resonance seems therefore localized structure can be induced. An example of towards the center of the windmill windmill rotation is shown in Fig. 7. The structure. No differences on either green dot indicates the position of the tip topography or spectroscopy were during the voltage pulse and the white observed between the four molecules lines indicate the position of the windmill structure before the manipulation. composing the windmill.

Pulse induced positioning of the windmill Rotational movements are mainly structures observed at positive voltage pulses (in more than 40% of the cases in contrast to The first step towards the development of less than 5% at negative voltages) with reliable atomic scale controlled molecular rotational angles of 15° (33%) or 30° machines is to understand the motion and (66%) in the clockwise or anticlockwise the intramolecular mechanics of a single direction. 999

project

AtMol

Fig. 7 > Rotation of a windmill nanostructure. Top Fig. 8 > Translation of a windmill after the panel: STM image of a windmill before application of several voltage pulses on a lateral manipulation. The green spot indicates the position group. Top panel: STM image of a windmill before of the tip during the application of a voltage pulse manipulation. The green spot indicates the of 2.3 V. Bottom panel: STM image of the same position of the tip during the application of the first windmill after the manipulation. The white lines voltage pulse of 2.3 V. Bottom panel: STM image of indicate the position of the windmill before the same windmill after the manipulation. The manipulation./ white lines indicate the position of the windmill before manipulation./

Considering that the windmill By applying the same kind of voltage nanostructures are chiral, it is interesting pulses outside the center of the windmill, to investigate if there is a rotational translational motions are induced. An preference connected to the chirality of example of such manipulation is shown in the windmill. However, out of the 200 Fig. 8. In such case, a series of four voltage measured rotational events, no significant pulses was applied, the first of them at the preference for one or the other rotation position of the green dot in the figure. The direction was observed. If there is a white lines indicate the position of the preferential rotation direction due to the windmill structure before the manipulation chirality of the molecules, as suggested by and help to visualize the movement of the a recent work on a single chiral molecular windmill. A single voltage pulse induces rotor [30], it will be rather small. From our typically a lateral jump of 3 Å, measurements, we can give an upper limit corresponding to the surface lattice for rotation directionality of about 5%. This constant of the Au (111) surface. To move confirms that molecule-surface chirality is the windmill over larger paths, series of only a necessary condition for voltage pulses are applied consecutively unidirectional rotation of an adsorbate. along a predetermined line on the surface. 101010

Importantly, by selecting the molecule to The experiment demonstrate that the which the voltage pulse is applied, the localized inelastic excitations of one of the direction of the manipulation can be ABP molecules building the windmill arte chosen. The windmill can be therefore sufficient to translate or to rotate the moved in four different directions (up, complete supramolecular windmill project down, right, left) depending on the position nanostructure without any internal of the tip on the nanostructure during the apparent structural change. Therefore,

voltage pulse. In the example of Fig. 8, the once a threshold voltage has been AtMol windmill moves down because the voltage reached, where tunneling in molecular pulse was applied on the lowest molecule. resonances is possible, the By selecting the sign of the voltage pulse, supramolecular structure can be moved. it is possible to switch between attractive or repulsive movements. A negative Conclusions voltage causes a movement of the The experiment presented here structure towards the tip, while a positive demonstrates a method to controllably voltage pulse induces the jump of the move supramolecular structures on a windmill away from the tip position. metal surface by STM and can help in More in detail, by changing the sign of the understanding the movements of weakly applied voltage, different types of motions bound molecules on metal surfaces. It can be selected. For negative voltages the provides a novel procedure to manipulate windmill moves in the direction away from individual nanostructures gently and the position of the tip. For positive purely electronically. The structures can voltages, two possible movement types be moved to the chosen positions on the could be observed. Most frequently, surface without the need for complex translational movements are induced. In intramolecular mechanical mechanisms. this case, the direction of the translation is STM at low temperature is at present the opposite to that for negative voltages; that only experimental method, which allows is, the center of the supramolecular both precise sub-molecular imaging of a structure moves toward the position of the single molecule and the possibility of tip. In some other cases, and only when manipulation and measurement of the the tip is positioned exactly in the center of electronic properties in clean and the nanostructure, the windmill rotates controlled conditions. Such properties can around its center when positive voltages be usefully applied to test molecular are applied. machines. In this case, the STM tip In order to understand the described supplies the driving forces necessary to manipulation events, the tip height traces power the nano-machine, which can be taken during the time interval of the driven electrically, by means of tunnelling voltage pulses can be systematically electrons or electric fields, or analyzed and the results compared with mechanically, by taking advantage of the dI/dV spectrum recorded over a chemical and Van Der Waals forces windmill at the same position where the between tip and molecule. voltage pulse has been applied for The manipulation of hydrogen bonded manipulation. It was observed in this way windmill nanostructures presented in this that the energy onset position of the article is an example of a purely resonances (Fig. 6) corresponds to the electrically driven nanomachine, where measured quantum yield, indicating the the repositioning of the nanostructure can presence of an inelastic input channel on be driven in a very precise and non- the windmill. 111111

destructive way. This work contributes to [18] P. Sautet and C. Joachim, Chem. Phys. understand the mechanics of molecular Lett. 185, 23 (1991). [19] W. H. Soe, C. Manzano, A. De Sarkar, N. nanostructures and to the development of Chandrasekhar, and C. Joachim, Phys. nanomachines controlled at the atomic Rev. Lett. 102, 176102 (2009). project scale. [20] F. Chiaravalloti, L. Gross, K.-H. Rieder, S. M. Stojkovic, A. Gourdon, C. Joachim, and Acknowledgements F. Moresco, Nat. Mater. 6, 30 (2007). [21] C. Manzano, W. H. Soe, H. S. Wong, F. AtMol Support from the ICT-FET Integrated Ample, A. Gourdon, N. Chandrasekhar, and C. Joachim, Nature Mater. 8, 576 (2009). Project AtMol, the European Union [22] N. Koumura, R. W. J. Zijlstra, R. A. van (ERDF) and the Free State of Saxony via Delden, N. Harada, and B. L. Feringa, the ESF Project 100087859 ENano and Nature 401, 152 (1999). ECEMP A2, the German Research [23] G. S. Kottas, L. I. Clarke, D. Horinek, and J. Michl, Chem. Rev. 105, 1281 (2005). Foundation (DFG) within the Cluster of [24] G. Vives, H.-P. J. de Rouville, A. Carella, J.- Excellence “Center for Advancing P. Launay, and G. Rapenne, Chem. Soc. Electronics Dresden”, the DFG-NSF Rev. 38, 1551 (2009). Project 11-568 is gratefully acknowledged. [25] L. Grill, K. H. Rieder, F. Moresco, G. Rapenne, S. Stojkovic, X. Bouju, and C. Joachim, Nat. Nano 2, 95 (2007). References [26] A. Nickel, J. Meyer, R. Ohmann, H.-P. Jacquot de Rouville, G. Rapenne, F. Ample, [1] F. Moresco, Phys. Rep. 399, 175 (2004). C. Joachim, G. Cuniberti, and F. Moresco, J. [2] K. Morgenstern, N. Lorente, and K.-H. Rieder, Phys.: Condens. Matter 24, 404001 (2012). Phys. Status Solidi B 250, 80 (2013). [27] T. Kudernac, N. Ruangsupapichat, M. [3] A. M. Moore and P. S. Weiss, Annu. Rev. Parschau, B. Macia, N. Katsonis, S. R. Anal. Chem. 1, 857 (2008). Harutyunyan, K.-H. Ernst, and B. L. Feringa, [4] D. M. Eigler and E. K. Schweizer, Nature Nature 479, 208 (2011). 344, 524 (1990). [28] Y. Shirai, A. J. Osgood, Y. Zhao, K. F. Kelly, [5] J. A. Stroscio and D. M. Eigler, Science 254, and J. M. Tour, Nano Lett. 5, 2330 (2005). 1319 (1991). [29] M. Böhringer, K. Morgenstern, W.-D. [6] L. Bartels, G. Meyer, and K. H. Rieder, Schneider, and R. Berndt, Angew. Chem. Phys. Rev. Lett. 79, 697 (1997). Int. Ed. 38, 821 (1999). [7] J. A. Stroscio, F. Tavazza, J. N. Crain, R. J. [30] H. L. Tierney, C. J. Murphy, A. D. Jewell, A. Celotta, and A. M. Chaka, Science 313, 948 E. Baber, E. V. Iski, H. Y. Khodaverdian, A. (2006). F. McGuire, N. Klebanov, and E. C. H. [8] D. M. Eigler, C. P. Lutz, and W. E. Rudge, Sykes, Nat. Nano 6, 625 (2011). Nature 352, 600 (1991). [9] J. Repp, G. Meyer, S. Paavilainen, F. E. Olsson, and M. Persson, Science 312, 1196 (2006). [10] . Swart, T. Sonnleitner, J. Niedenführ, and J. Repp, Nano Lett. 12, 1070 (2012). [11] M. Lastapis, M. Martin, D. Riedel, L. Hellner, G. Comtet, and G. Dujardin, Science 308, 1000 (2005). [12] Y.-Q. Xu, J. Zhang, B.-K. Yuan, K. Deng, R. Yang, and X.-H. Qiu, Acta Phys. Chim. Sin. 26, 2686 (2010). [13] B. C. Stipe, M. A. Rezaei, and W. Ho, Science 279, 1907 (1998). [14] S.-W. Hla, L. Bartels, G. Meyer, and K.-H. Rieder, Phys. Rev. Lett. 85, 2777 (2000). [15] U. G. E. Perera et al., Nat. Nano 8, 46 (2013). [16] A. Nickel, R. Ohmann, J. Meyer, M. Grisolia, C. Joachim, F. Moresco, and G. Cuniberti, ACS Nano 7, 191 (2012). [17] M. Yu et al., ACS Nano 4, 4097 (2010). 121212

nanoresearch

Coronene: A Single Molecule Atom Counter

C. Manzano1, W. H. Soe1, M. Hliwa2, Recently it has been demonstrated that a M. Grisolia2, H. S. Wong1 and C. Joachim1,2 trinaphthylene molecule can function as a NOR logic gate [9,10] by using single 1 IMRE, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602, Singapore Au adatoms as classical logical inputs. 2 GNS-CEMES & MANA Satellite, CNRS, The molecule converts the inputs 29 rue J. Marvig, 31055 Toulouse Cedex, France classical on/off information in quantum information leading to a controlled shift of

trinaphthylene's electronic quantum To contact selected peripheral π bonds states which conforms with the output of of a single Coronene molecule, gold a NOR logic gate. The rationale of this adatoms were manipulated one by one approach is that it is better to embed on a Au(111) surface with the tip of a higher level functionalities within one scanning tunnelling microscope to form single molecule rather than to pursue the Au -Coronene complexes. Tunnelling n chemical synthesis of large molecules electron spectroscopy and differential having the shape or functions of single conductance mapping show how the electronic circuit components. electronic ground state of the Aun- Coronene complexes is shifted in Intramolecular quantum behaviours are energy concomitantly with the number also rich enough to implement simple of gold atoms coordinated to the analogue functions inside a single Coronene molecule. By simply following molecule without forcing the molecule to the linear energy downshift of the Aun- have the shape of an analogue electrical Coronene’s ground state the number of circuit. To demonstrate this, we have interacting atoms can be counted selected the Coronene molecule and demonstrating that a Coronene manipulated its ground electronic state molecule can function as a single by contacting its π electronic system with molecule atom counter. single Au atoms. A Coronene molecule is

able to count one by one the number of Introduction Au atoms interacting with its π system. The output reading of this counter is Molecules are often considered as performed by recording the tunnel potential building blocks to be used in the current intensity passing through very fabrication of future generations of specific positions on the Coronene π electronic circuits with components system. miniaturized down to atomic scale. Molecules can be chemically designed to In this article, this counting effect is mimic standard electronic circuits by demonstrated experimentally using a low using chemical groups designed to temperature STM, single atom and function like the basic elements of a molecule STM manipulations, differential digital electronic circuit [1-6]. It has also conductance (dI/dV) constant current been proposed to abandon classical imaging and dI/dV spectroscopy. All the electronic circuit design and to perfom images were calculated using the Elastic Boolean logic operations by manipulating Scattering Quantum Chemistry technique the electronic quantum states of a single in the framework of the Extended Hückel custom designed molecule [7,8]. Molecular Orbitals approximation (EHMO- ESQC) [12]. 131313

Model System three Au atoms, there are three fully equivalent interacting configurations To count classically using a quantum possible e.g. (1,3,5). Once a given phenyl system, let us consider a model system is selected for the first Au input, the other project with two non degenerate electronic states, atoms must be contacted respecting |g> being the ground state of the counter specific input positions for the Coronene and |i> the state corresponding to one to work as a counter. The output reading input. When |i> and |g> are electronically

AtMol position around the Coronene external coupled each state undergoes an energy phenyl crown must also be selected shift, |g> shifts down and |i> shifts up, with depending on this initial input choice. ∆ being the initial energy difference between those 2 states. For a small coupling α as compared to ∆, |g> still keeps its ground state characteristics and becomes |G> with an energy shift downwards of α2/∆ likewise |i> becomes |I> with a α2/∆ upward energy shift. When N identical |i> states are coupled to |g>, the resulting new |G> ground state is shifted by (∆√(∆2+4Nα2))/2. The ground state |G> will be linearly shifted down in energy by Nα2/∆α as a function of N and for α<<∆. Therefore, the |G> energy position can be used to count the number of identically coupled |i> states. Notice that this counter can saturate when 2√(N)α is not negligible compared to ∆. But this limitation depends on how each |i> state is spatially coupled to |G> as discussed below.

To demonstrate experimentally this counting effect, we have manipulated the ground state (HOMO) of a Coronene molecule by bringing single Au atoms in

interaction with Coronene’s π system. A Coronene molecule has six peripheral Fig. 1 > (Top) Characteristic dI/dV spectra phenyl rings (see Coronene’s model in recorded on a Coronene molecule (in black) and Fig. 1), then there are six equivalent sites on a Au1-Coronene complex (in red). The energy shift of Au1-Coronene ground state accessible for contacting one Au atom. resonance (HOMO) with respect to Coronene's There are three different possible HOMO is clearly observed. (Bottom) Images configurations to contact two atoms: they corresponding to the STM topography, dI/dV could be bind to two nearest neighbouring map and ESQC calculated conductance map of phenyl rings like (1,2) or (2,3), interact Coronene and Au1-Coronene respectively with nonadjacent phenyl rings like (1,3) or (image size: 2 nm x 2 nm). The ESQC dI/dV (1,5) or be in diametrical opposite maps were both calculated at the energy corresponding to the HOMO resonances of positions like (1,4), here each of the Coronene and Au1-Coronene./ numbers in parenthesis indicates the external Coronene phenyl each single Au

atom is contacted to (see Fig. 2). For 141414

Results and Discussion constant current images calculated at the HOMO resonance match perfectly the A sub-monolayer of Coronene molecules experimental images, also shown in Fig. (C24H12, Sigma-Aldrich 99.99% purity) 1. The calculated images enable to was deposited on the surface of a identify the 6 lobes observed on the project Au(111) mono-crystal using free experimental dI/dV images, these lobes evaporation. During the evaporation, the correspond to the outer phenyls of substrate temperature was kept below Coronene. AtMol 80°C. After the molecules sublimation the sample was cooled down using liquid To form the Aun-Coronene complexes helium and transferred to the STM the Au adatoms and the Coronene chamber. All STM imaging, manipulation molecule were brought together into and spectroscopy measurements were electronic interaction using STM done at ~7 K. manipulation in constant current mode. Atom manipulations done to form atom- Prior to any experiment an oxide free molecule contacts show that manipulated electrochemically etched Tungsten tip is single gold atoms arrange selectively in a gold coated by indenting it in the Au(111) conformation in which a gold atom is substrate following the method described always underneath an outer phenyl ring. et al by Hla [13]. Spectra are taken to Attempts to obtain single atom contacts test whether the tips are in fact metallic at other positions of Coronene’s external and fit for obtaining spectroscopic data. phenyl rings as well as underneath Only tips whose spectra taken on a clean Coronene’s center resulted in unstable Au terrace showed the resonance conformations and it was not possible to corresponding to the surface state of investigate their effect in the Au atom Au(111) were used in our experiments. counting process.

Constant current STM images taken after The energy shifts of Coronene’s ground depositing Coronene molecules show state as a function of the number N of that the molecules adsorbed both on interacting Au atoms were tracked by Au(111) terraces and at step edges. the systematically recording dI/dV spectra on characteristic herringbone reconstruction lobes away from the contacted outer of Au(111) can also be observed. In the phenyls. After contacting one Au atom to topographic STM images, single any of Coronene’s six outer phenyl rings Coronene molecules are imaged like a 6 its dI/dV spectrum presents a shift of the branched star as presented in Fig.1. HOMO (ground state) resonance by Characteristic dI/dV spectra of single nearly 200 mV with respect to a bare Coronene molecules were obtained by Coronene’s HOMO (see Fig. 1). The positioning the STM tip over a molecule's experimental and ESQC calculated center and over any of its 6 outer lobes. constant current dI/dV images of a Au1- The main features of dI/dV spectra taken Coronene complex match perfectly from outer lobes are a resonance peak at indicating that the interaction of a single -1.4 V and a broad tail at positive bias Au atom with the Coronene has broken showing no distinctive resonance. the HOMO symmetry through the local Furthermore dI/dV spectra taken at the deformation of the Coronene skeleton at molecule center appear featureless. On the Au interaction location. This a constant current dI/dV image recorded deformation was already observed for at -1.4 V, the central Coronene phenyl the interaction of a trinaphthylene ring appears featureless while six molecule with a single Au atom [9,10]. external lobes are clearly imaged. ESQC 151515

The downshift in energy relative to the native Coronene’s HOMO is observed for each subsequent addition of single Au atoms. As project presented in Fig. 2, the dI/dV spectra recorded for the different Au2-Coronene complexes (1,2),

AtMol (1,3), (1,4) show its negative resonance peak shifted by 400 mV relative to the HOMO of a bare Coronene. The (1,3,5) Au3- Coronene complex shows a resonance peak shifted also by nearly 200 mV as compared to the Au2-Coronene complexes HOMO, that is a 600 mV spectral shift from Coronene’s HOMO (See Fig. 2).

Fig. 2 > Spectra recorded on a Coronene The electronic structure and molecular molecule and on each of the Aun-Coronene orbitals of Aun-Coronene have been complexes investigated (n: 1, 2, 3). These spectra calculated as a function of N using the show how each additionally contacted gold MOPAC2009 semi-empirical quantum atom resulting in a Aun-Coronene produces a chemistry code in the framework of the further spectral shift with respect to Au(n-1)- Potential Model-parametrization-6 Self Coronene. At the top: STM topographic images Consistent Field approach (PM6-SCF) (2 nm x 2 nm) of each Aun-Coronene complex the spectra were taken from, the exact location [11]. Electronic structure diagrams of the each spectrum is taken on the complexes is Aun-Coronene complexes clearly show indicated in the images. The hexagons with the incremental energy shifts of their numbered circles at the top of each STM image electronic ground states concomitantly represent Coronene's outer phenyl rings. These with each additional gold atom diagrams represent each of the input conformations, their corresponding spectrum is contacted. indicated each with an arrow. The highlighted circles indicate the phenyl rings the gold atoms Molecular orbital calculations done for all were coordinated to./ Aun-Coronene complexes reveal that the Single Occupied Molecular Orbital The calculations enable to identify the (SOMO) of a bare Au1-Coronene origin of the deformation which arises complex is nearly a pure Au 6s atomic from the two carbon atoms at the end of orbital. The location of this 6s Au orbital the phenyl ring directly interacting with in the Coronene HOMO-LUMO energy the Au atom, due to their interaction the gap is responsible for the stabilization of two carbon atoms are pushed up the Coronene HOMO. When Coronene together with their corresponding interacts with two Au atoms, the HOMO hydrogen atoms. This deformation and LUMO of the Au2-Coronene complex breaks the delocalization of Coronene’s are essentially the symmetric and anti- π system explaining how both the symmetric superposition of the two Au 6s experimental and the calculated images orbitals. In this input conformation are showing only 4 lobes instead of the 6 Coronene’s native HOMO is shifted lobes imaged for a bare non deformed down further in energy as compared to physisorbed Coronene. 161616

the Au1-Coronene complex and becomes using PM6-SCF. There is an excellent Au2-Coronene’s HOMO-1. For the Au3- agreement between the experimental Coronene complex, the three Au 6s and the calculated shifts as a function of orbitals give rise to three molecular the number of Au atoms interacting with orbitals HOMO, SOMO, and LUMO. a Coronene molecule. This confirms that project These states are also located within the the presence of an Au 6s orbital in the bare Coronene HOMO-LUMO energy region of the HOMO-LUMO energy gap

gap and stabilized even more stabilizes linearly the Coronene HOMO. AtMol Coronene’s native HOMO which is again Conclusions the HOMO-1 of the Au3-Coronene complex. With a good selection of the Herein we have demonstrated that STM metal atom inputs and of their location manipulated Au adatoms contacted to relative to the Coronene π system, the selected phenyl rings of a single initial HOMO of the Coronene can be Coronene molecule result in an energy pushed down in energy following a linear shift of the ground state of the Aun- law as given by the simple model Coronene complexes. This energy shift described above. is linear as a function of the number of

contacted Au atoms and therefore can be used to count the number of atoms input, hence demonstrating that Coronene can function as a single molecule atom counter.

We have also calculated how the counting effect stands using a larger Aun- circumcoronene series where 6 outer phenyls are available on the circumcoronene external phenyl crown. Indeed the circumcoronene HOMO is downshifted in energy as a function of the

Fig. 333 >>> Plots showing the linear spectral number of Au atoms interacting with its π downshift of Aun-Coronene's HOMO peak as system. Even if the number of interacting function of the contacted n gold atoms. The Au reaches a maximum of 6, the shift is electronic structure of Aun-Coronene complexes linear with no apparent saturation in free space calculated using PM6-SCF show because in the HOMO state, the orbital the same spectral shifts observed in the weight of the outer phenyl decreases due experimental spectra. The corresponding ∆E shift for each complex is plotted. Both to this ground state quantum experimentally measured (in blue) and normalization. For even larger circular π calculated shifts (in red) follow the same conjugated systems corresponding to a spectral behaviour dependency with respect to nano-graphene like macromolecule disk, the number of gold atoms input. / the HOMO normalization will stabilize the

The recorded dI/dV spectral shifts of counting effect since the number of central phenyl rings will increase faster than the selected input conformations for all Aun- Coronene complexes showing the number of available phenyls on the last counting function of Coronene are outer crown of the disk. It remains to be plotted in the Fig. 3 and compared with determined how fast this effect occurs as the spectral shifts extracted from a function of the disk diameter to estimate electronic structure diagrams calculated the maximum number of Au atoms a nano-graphene disk can count. 171717

Acknowledgements [7] Haruka Goto, Hiroyuki Katsuki, Heide Ibrahim, Hisashi Chiba, Kenji Ohmori, We acknowledge financial support from Nature Physics 7, 383–385 (2011). the Agency of Science, Technology and [8] N. Jlidat, M. Hliwa, C. Joachim, Chem. Phys. Lett. 452 (2008) 270. project Research (A*STAR) for the Visiting [9] W. -H. Soe, C. Manzano, N. Renaud, P. de Investigatorship Program (Phase III): Mendoza, A. De Sarkar, F. Ample, M. Hliwa, AtomTech Project and the AtMol (2011- A. M. Echavarren, N. Chandrasekhar, and 2014) European Commission integrated C. Joachim, ACS NANO 5 (2), pg. 1436- AtMol project. 1440 (2011). [10] W. -H. Soe, C. Manzano, A. De Sarkar, F. Ample, N. Chandrasekhar, N. Renaud, P. References de Mendoza, A. M. Echavarren, M. Hliwa, and C. Joachim, PHYSICAL REVIEW B 83, [1] A. Aviram, M. Ratner, Chem. Phys. Lett. 29 155443 (2011). (1974) 277. [11] J. J. P. Stewart, J. Mol. Model. 13, 1173 [2] F.L. Carter, Physica D 10 (1984) 175. (2007). [3] C. Joachim, J.K. Gimzewski, Chem. Phys. [12] P. Sautet and C. Joachim, Chemical Lett. 265 (1997) 353. Physics Letters, 185, 23 (1991). [4] J.C. Ellenbogen, J.C. Love, IEEE 88 (2000) [13] Hla S.-W.; Braun K.-F.; Iancu V.; 386. Deshpande A. Nano Letters 2004, 4, 1997- [5] C.A. Stafford, D.M. Cardamone, S. 2001. Mazumdar, Nanotechnology 18 (2007) 424014. [6] H. He, R. Pandey, S.P. Karna, Nanotechnology 19 (2008) 505203.

181818 www.atmol.eu

Atomic Scale and single Molecule Logic gate Technologies

nano research

Measuring the conductance of single molecular wires as a function of the electron energy

Matthias Koch1, Francisco Ample2, electronic and mechanical properties. It Christian Joachim2,3 and Leonhard Grill1,4 consists of sp2-hybridized carbon atoms

1 arranged in a honeycomb lattice [1]. Due Dept. of Physical Chemistry, FritzHaberInstitute of the MaxPlanckSociety, Berlin, Germany to the massless propagation of the 2 Institute of Materials Research and Engineering electrons through the graphene flake (IMRE), Singapore they mimic relativistic particles and can 3 Nanosciences Group, CEMESCNRS, Toulouse, be described by the Dirac equation and France 4 Dept. of Physical Chemistry, University of Graz, not by the Schrödinger equation. The Heinrichstrasse 28, Austria electrons lose their mass at the Dirac points and their carrier mobility can be as leonhard.grill@unigraz.at high as 100,000 cm2V-1s-1. One-

We have measured the conductance of dimensional graphene stripes, so-called single graphene nanoribbons as a nanoribbons, are therefore of particular continuous function of their length, thus interest as molecular wires. Specifically, determining the current decay along the their electronic structure, and molecular wire, in combination with consequently charge transport theoretical calculations. After on- properties, depends on the precise surface polymerization of the atomic structure of the graphene nanoribbons on a Au(111) surface, the nanoribbon [2, 3]. Here we present tip of a scanning tunneling microscope conductance measurements of a single is used to pull single polymers off the narrow graphene nanoribbon by pulling it surface at cryogenic temperatures. In from a metal surface. Detailed this way, individual molecular chains understanding of charge transport are electrically contacted between two through a single molecule requires not electrodes, the tip and the surface. By only such length-dependent conductance systematically changing the bias measurements, but also a systematic voltage between the electrodes, the variation of the electrode potentials conductance is investigated as a relative to the molecular electronic function of the electron energy and the states. We show for the first time how the charge transport properties are directly conductance properties of a single correlated with the electronic structure molecule are correlated with its of the molecular wires, in agreement electronic states which are spatially with theory. In a very particular mapped via scanning tunneling configuration, the nanoribbons adapt an spectroscopy before the conductance almost straight shape and experimental measurement. The tunneling decay evidence for a pseudo-ballistic transport length is measured over a wide bias regime is found at the single-molecule voltage range from the localized Tamm level, as predicted by theory. states over the gap up to the delocalized occupied and unoccupied electronic Introduction states of the nanoribbon. The major part of our results has been published Graphene fascinates scientists all over recently. We furthermore show how the the world because of its unique conductance depends on the precise 212121

atomic structure and on the bending of β is the inverse decay length. The the molecule in the junction, revealing inverse decay length depends on the the importance of the edge states and a position of the HOMO and LUMO relative planar geometry [4]. Finally, we provide to the Fermi level EF as well as on their project experimental evidence for pseudo- energy difference Eg and on the effective ballistic transport through single mass of the electron in the tunnelling molecules, i.e. an ideal transport regime junction. A large Eg leads to high β

AtMol where the current does not decay with values and consequently to a low the molecular wire length. junction conductance, for instance for alkane chains. If the molecule in the Electron transport through a molecule is junction exhibits electronic states that are fundamental in chemical and biological located at E another transport regime is processes [5]. It depends on various F, active. In such a case, β becomes very parameters: the electronic structure of small, for example with the d states of an the molecule, its interaction with the organo-metallic compound [10] or the π junction electrodes and the energy states of a short organic molecular wire position of the molecular electronic [14]. Consequently, a pseudo-ballistic states with respect to the electrodes. The transport regime with still G < 2e2/h (i.e. relation between the molecular electronic the quantum of conductance) occurs, structure and the exponential decay of since the electronic structure of the the electron transfer rate with length has molecular wire differs from that of the been studied experimentally in solution metallic electrodes. [6]. Charge transport has also been investigated for various lengths of We investigate graphene nanoribbons molecular wires when contacting their (GNRs) as their electronic structure can termini to electrodes, either by be controlled via their width [2] and edge comparing different molecules [7-10] or structure (zig-zag or armchair) [3]. In by continuously changing the electrode contrast to top-down fabrication of distance along the molecule [11, 12]. In GNRs, which lack control over the ribbon one study [13] the electrodes bias width and/or edge structure at the atomic voltage has been varied, but the electron scale, bottom-up on-surface energy remained always at low voltages polymerization [15, 16] produces in the gap between the ground state (i.e. precisely defined molecular structures. A HOMO) and the first excited states (i.e. well-defined chemical structure is LUMO) of the molecular wire, while required for charge transport several thousands of molecules were measurements as structural defects present in the junction at the same time. modify the electronic structure and However, the conductance of a single reduce the conductance [17]. molecule can differ significantly compared to the conductance per Experimental details molecule in an ensemble. Therefore, measurements are necessary at the level To produce GNRs with an armchair edge of a single and well characterized structure we have used 10,10'-Dibromo- molecule. 9,9'-bianthryl molecules (Fig.1a). For the fabrication two heating steps are required: The conductance G of a molecular wire On-surface polymerization takes place at decays exponentially with its length d 200°C, where non-planar anthracene -βd and can be described by G(d) = Go e , oligomers are formed (Fig.1b) and where Go is the contact conductance and cyclodehydrogenation at 400°C (Fig.1c). 222222

created. A modified low temperature STM from Createc was used and all measurements were taken at about 10 K. project Results and Discussion

The graphene nanoribbons appear as straight stripes with an apparent AtMol height of 1.85 ± 0.12 Å (Fig.1d). Their contour is rather homogeneous with characteristic finger-like lobes at both termini (Fig.1e), independent of the ribbon length, if small bias voltages are used. These features are typical for the intact molecule when the Br atoms have already been dissociated. To study the electronic Fig. 1 > Chemical structure of the 10,10'- origin of the bias-dependent appearance, Dibromo-9,9'-bianthryl molecules (a) which are dI/dV spectra, which are known to reflect connected to oligomers (b) after the first heating step and to graphene nanoribbons (c) after the the electronic transparency of the second heating step. STM images (3 x 7 nm2) of junction[19], were recorded as a function a single graphene nanoribbon at different bias of the voltage. At the termini a peak at voltages: -1.55 V (d), +0.05 V (e). STM image about 30 meV above the Fermi level is 2 (3 × 7 nm ) (f) and corresponding dI/dV curves resolved, which completely vanishes at (g) taken at different positions (1–10) along a the centre of the ribbon (Fig.1f-g). The single nanoribbon, revealing the presence of a peak at the ribbon termini that decays within main difference of these two areas is the 10–15 Å along the ribbon./ structure of the ribbon edge, which is zig- zag at the termini and armchair sideways. We assign this feature to the Experiments were carried out under ultra- theoretically predicted Tamm states [20], high vacuum conditions with a base -10 which originate from the specific pressure of 10 mbar. Before depositing arrangement of the carbon atoms at zig- the molecules the Au(111) sample was zag edges and are absent at armchair cleaned by Ne ion sputtering (E = 1.5 keV) edges. and subsequent annealing to 750 K. A Knudsen cell with a temperature of about To measure the conductance of a single 470 K was used to evaporate the 10,10'- GNR, it was lifted off the surface by Dibromo-9,9'-bianthryl (DBDA) molecules, controlled STM pulling [12] after controlling the temperature by a attaching one end of the ribbon to the tip. thermocouple. The sample was typically Before and after a pulling experiment the at 470 K during deposition (deposition at GNR is characterized by imaging and room temperature results in single spectroscopy. While the tip is retracted, molecules), Br dissociates and anthracene the effective transport length through the oligomers are produced, according to the ribbon (indicated by an arrow in Fig.2a) on-surface polymerization process by is modified, because the current is only dehalogenation [15]. Annealing the passing through the part of the GNR that Au(111) sample further for 5 min to 670 K does not interact with the surface. If the initiates cyclodehydrogenation [18] and molecule is successfully lifted from the fully aromatic graphene nanoribbons are surface, the slope of the measured STM 232323

decreases (Fig.2b) and hence the conductance rises. The decay of the current for both polarities is project in the same range which points to a similar charge transport mechanism.

AtMol Moreover, all pulling curves in Fig.2b are very smooth, in contrast to the local bending of a chain- like polyfluorene that causes characteristic oscillations in the current curve [12]. These constant slopes are an evidence for a continuous bending of the ribbon in the junction, this is confirmed by calculations.

The change in conductance is related to the electronic states of Fig. 2 > a) Schematic of the STM pulling experiment (arrow the GNR. Therefore, we indicates tunneling current). A characteristic current signal measured dI/dV spectra during the pulling sequence is shown in the right panel. b) of a single ribbon (Fig.3a), Current as a function of tip height for different experiments (zero tip height refers to tip–surface contact). Different ribbon similar to Fig.1g but for a lengths were used in the experiments, resulting in equivalent larger voltage range. We conductance properties./ find two intense peaks at -1.1 V and +1.6 V, which current signal I(z) (i.e. higher conductance) are assigned to the ground and to the is smaller than for the vacuum junction. first excited state (approximately HOMO From the linear curve in the logarithmic and LUMO) of the molecule, indicating plot (and thus exponential decay), we find Eg = 2.7 eV. In order to correlate the that electron transport occurs in the electronic states to the charge transport tunnelling regime and obtain a β value of mechanisms conductance measurements 0.45 ± 0.06 Å-1 for a low bias voltage of were done for bias voltages between around 0.4 V (Fig.2b). In comparison, a -1.8 V and +2.4 V, hence covering all steeper curve and thus larger β value of relevant electronic states (HOMO, 0.76 ± 0.04 Å-1 is found for the anthracene LUMO and Tamm states). Independent of oligomer (Fig.2b; taken at 50 mV bias the bias voltage the current decays voltage), i.e. the same molecules before exponentially with respect to the tip height planarization (Fig.1b), due to their lower (as in Fig.2b). This provides evidence that electron delocalization and larger Eg value. tunnelling is always the charge transport The electrode electron energies are shifted and allows us to extract the inverse decay with respect to the GNR depending on length β as a function of the bias voltage the applied bias voltage. At a bias (Fig.3a). Three different regimes can be voltage of -1.4 V and +1.8 V the slope distinguished: At low bias voltages (up to 242424

about 1.6 V), β values around 0.45 ± 0.1 Å-1 at negative bias voltages. This is 0.06 Å-1 are found. According to our in qualitative agreement with calculations calculations, efficient charge transport (Fig.3b) as the decrease in the β value takes only place in this regime if the matches with the onset of the HOMO Tamm states at opposite ends overlap and LUMO molecular orbitals. project and thus contribute to the current Experimental dI/dV conductance maps (Fig.4a). This is only the case for short show that the HOMO as well as the ribbons with a length of around 2-3 nm. LUMO are delocalized along the ribbon AtMol In contrast to the armchair edge structure edges (Fig.4c and d). In a tunnelling here, a zig-zag edge strongly improves transport regime, the inverse decay the conductance, because the Tamm length depends on the electron energy, states cause electron delocalization the molecular energy levels (Eh and El along the ribbon and thus no current are the positions of HOMO and LUMO, decay is observed with the distance respectively) and the effective mass m* (Fig.4a). of the electrons [21]

The conductance improves if the electron * 2m (E) (E − Eh )(El − E) (1) energy coincides with the position of the β ()E = 2 E molecular electronic resonances in the h g dI/dV spectrum (at -1.1 eV and +1.6 eV, β is therefore expected to decrease as respectively), resulting in lower β values soon as the electron energy matches a of 0.18 ± 0.03 Å-1 (at 1.8 V to 2.4 V) and molecular energy level. a continuous reduction down to about

Fig. 3 > Conductance dI/dV spectra (top, a and b; all in a pulling geometry) of a graphene nanoribbon and its inverse decay length b (bottom, a and b) for different bias voltages from various nanoribbons used in the experiment (a) and ESQC calculations (b). a) Top: dI/dV spectrum for a nanoribbon in a pulling geometry. Bottom: Error bars reflect the precision in b determination in each individual I(z) curve (at a constant bias voltage and contributing as one data point); a dashed line is drawn to guide the eye. The experimental β values, determined from many nanoribbons, need to be reduced (by 10–15%) to obtain the real values, because the STM tip height is slightly smaller than the effective molecular length in the junction. b) Top: The calculated dI/dV curve (in a pulling geometry) shows the Tamm states resonance doublet, split due to the nanoribbon curvature during pulling. Bottom: The ideal planar b curve (red) is presented for comparison with the curve in an STM pulling geometry (black)./

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It is important to note that ideal (pseudo- reduced. The 48 Å nanoribbon has been ballistic) electron transport at high bias pulled up twice, revealing the same voltages, where the current does not result in both cases. This reproducibility decay with the length of the molecular of the reduction of β for very large tip project wire (i.e. β = 0 Å-1), takes only place for a heights supports our interpretation. The planar ribbon (red curve in Fig.3b). two curves do not only become flat (i.e. However, values of around 0.1 Å-1 are no current decay), but also slightly

AtMol calculated for a pulling configuration increase at the end. This behaviour is (black curve), in agreement with the expected as the β value of approximately experimental values between 0.1 and 0 Å-1 for a planar nanoribbons leads, in 0.2 Å-1 at high bias voltages (Fig.3a). Our combination with the considerable calculations show that this is due to the contact conductance values Go (towards non-planar configuration of the ribbon the tip and towards the surface), to a during the pulling process because such current value that exceeds the one for a a deformation perturbs the electronic curved ribbon at these large tip heights. delocalization along the molecule Accordingly, the current curve can even (Fig.3b). On the other hand, we also find increase in such a configuration as evidence for this particular charge observed in Fig.4b. transport regime in our experiments if the tip-nanoribbon contact holds up to very large pulling heights. In such a particular case, the curvature of the nanoribbon must be reduced for geometrical reasons and therefore a more planar configuration is achieved. Although the exact ribbon geometry cannot be determined from the STM experiments, a comparison of the nanoribbon length on the surface and the pulling height of the STM tip gives some insight: The ideal straight nanoribbon configuration should be reached if these two values are equal (not taking into account the small error in the determination of

the tip height, while the nanoribbon Fig. 444 >>> Calculated pulling curves for low bias length can be measured very precisely voltages of a GNR with arm chair (Case I) and from the STM images before pulling). In zig-zag edges (Case II). b) Three pulling curves agreement with these considerations, we of two different graphene nanoribbons. The indeed observe a reduction of β for very length of the nanoribbons is 98 Å and 48 Å. In both cases a bias voltage of +1.8 V is applied. large tip heights that approach the values c,d) Experimental dI/dV conductance maps (all of the nanoribbon length. This is shown 5×5 nm2 in size) of a defect free nanoribbon in Fig.4b for two nanoribbons of different terminus at different bias voltages: (c) -1.15 V lengths where the β value is strongly (HOMO level) and (d) +1.6 V (LUMO level)./ 262626

Conclusions References

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